1. Field of the Invention
The present invention relates to a method for the contamination-free comminution of semiconductor material. Furthermore, the invention relates to an apparatus for carrying out the method.
2. The Prior Art
At the beginning of the production of many semiconductor products, it is necessary to provide semiconductor material in molten form. In most cases, the semiconductor material is melted for this purpose in crucibles or the like. Molded bodies are then cast, or crystals are then pulled from the melt by known methods. These are the basic material for products such as, for example, solar cells, memory chips or microprocessors. If the semiconductor material to be melted is in the form of solid large-volume bodies such as, for example, in rod form after a gas-phase deposition, it has to be comminuted for the melting process in the crucible. Only in this way is it possible to utilize the crucible volume efficiently and to achieve short and energy-saving melting times as a result of the large surface of the melting charge which has been introduced in small particles.
During the comminution, care has to be taken to ensure that the surfaces of the fragments are not contaminated with impurities. In particular, contamination with metal atoms is to be regarded as critical, since the latter can alter the electrical properties of the semiconductor material in a harmful way. If the semiconductor material to be comminuted is comminuted, as usually has been done in the past with mechanical tools such as, for example, steel crushers, the fragments have to be subjected to a complex and cost-intensive surface cleaning before melting.
According to DE-3,811,091 A1 and the corresponding U.S. Pat. No. 4,871,117 it is possible to decompact solid, large-volume silicon bodies in such a way that the mechanical comminution is possible even with tools whose working surfaces are composed of non-contaminating, or only slightly contaminating substances, such as silicon, or nitride ceramics or carbide ceramics. The decompacting is achieved by creating a temperature gradient in the silicon piece to be broken as a result of heat action from the outside and establishing a surface temperature of 400.degree. C. to 1400.degree. C., and rapidly reducing the latter by a value of at least 300.degree. C. so that the temperature gradient at least partially reverses. To create the temperature gradient, the solid charge has to be placed in a furnace and heated. This method has, however, the disadvantage that, during the heating phase, the diffusion of impurities adsorbed at the surface of the semiconductor material is set in motion and/or accelerated. In this way, the impurities from the surface enter the crystal structure of the semiconductor material and consequently escape the cleaning measures which are able to remove only impurities near the surface. In addition, in the method mentioned, a contamination of the semiconductor material by impurities given off by the furnace material during the heating is virtually unavoidable.